Abstract

Dynamics of single molecules in liquids, inspected in the picosecond time scale by means of spectroscopic measurements or molecular-dynamics (MD) simulations, reveals a complex behavior which can be addressed as due to local confinement (cage). This work is devoted to the analysis of cage structures in liquid benzene, obtained from MD simulations. According to a paradigm proposed for previous analysis of atomic and molecular liquids [see, for example, A. Polimeno, G. J. Moro, and J. H. Freed, J. Chem. Phys.102, 8094 (1995)], the istantaneous cage structure is specified by the frame of axes which identifies the molecular configuration at the closest minimum on the potential-energy landscape. In addition, the modeling of the interaction potential between probe molecule and molecular environment, based on symmetry considerations, and its parametrization from the MD trajectories, allows the estimation of the structural parameters which quantify the strength of molecular confinement. Roto-translational dynamics of probe and related cage with respect to a laboratory frame, dynamics of the probe within the cage (vibrations, librations, re-orientational motions), and the restructuring processes of the cage itself are analyzed in terms of selected time self-correlation functions. A time-scale separation between the processes is established. Moreover, by exploiting the evidence of fast vibrational motions of the probe with respect to the cage center, an orientational effective potential is derived to describe the caging in the time scale longer than .

Received 20 March 2005Accepted 13 July 2005Published online 27 September 2005

Acknowledgments:

The work has been supported by a FIRB-2001 Project entitled “Dinamica di liquidi molecolari” (Grant No. RBAU01FEH2), by the Italian Ministero dell’Istruzione, dell’Università e della Ricerca and by the European Union (Grant No. RII3-CT-2003-506350).

Article outline:I. INTRODUCTIONII. IDENTIFICATION OF THE CAGE FRAMEIII. ROTO-TRANSLATIONAL DYNAMICS IN LF AND MOLECULAR CAGINGIV. THE ROTO-TRANSLATIONAL CAGE POTENTIALA. Modeling of the RT potential close to the minimum-energy pointB. Parametrization of the RT potentialV. PROBE DYNAMICS REFERRED TO CF, CAGE RESTRUCTURING, AND TIME-SCALE SEPARATIONVI. THE EFFECTIVE ORIENTATIONAL POTENTIAL IN THE PERSISTENT CAGEVII. CONCLUSIONS AND PERSPECTIVES